This invention relates generally to techniques for the interconnection of electronic equipment modules and, more particularly, to the interconnection of communication equipment modules through a backplane. A common technique for interconnecting electronic equipment modules is by means of a set of interconnection conductors placed over a common return path, usually referred to collectively as a bus. Multiple equipment modules are connected to the bus, usually in a removable fashion, and a standard connection interface is defined such that each module makes electrical contact with appropriate conductors forming the bus. The principal function of the bus is to carry data or other signals from one module to another, or to distribute data from one module to multiple other modules, or to distribute non-data signals required for normal operation, such as power or clocking signals. In a configuration familiar to personal computer users, a bus may be formed in a "motherboard," on which other components are also mounted, and into which various "daughterboard" circuit cards are plugged. An interconnecting bus may also be referred to as a "backplane," because the individual conductors of the bus are formed as metal traces on the rear face of a circuit board, such as the motherboard used in personal computers. The backplane is formed as a collection of signal conductors over a common reference plane. A signal's return current is carried in the reference plane, making this an unbalanced transmission system. The most common example of this is called a microstrip transmission line. A backplane commonly provides access to the signal conductors with the use of one or more connectors.
A well known problem associated with electronic equipment is electromagnetic radiation. The degree to which electronic equipment emits electromagnetic radiation is stringently regulated by various governmental agencies. In the United States, the Federal Communications Commission (FCC) promulgates regulations to limit the amount of radiation emitted from electronic equipment, and to provide for routine testing of manufactured products to ensure compliance. Similar administrative bodies perform the same function in Canada, Europe and other parts of the world. Electromagnetic radiation is undesirable because it may interfere with radio and television transmission through the atmosphere or with the operation of other nearby electronic equipment, and it may have harmful physiological effects. In general, there are two types of solutions for reducing electromagnetic radiation from electronic equipment: those that increase shielding around the equipment and those that control the frequency spectrum of signals responsible for the emission of radiation. A basic shielding technique is to place the equipment in encompassing metal enclosures and install radio-frequency (rf) gaskets to minimize emissions through necessary openings in the enclosures. The encompassing conductive enclosure forms a Faraday shield which reduces radiated power by forcing the electric field component of an electromagnetic wave to be nearly zero at the surface of the conductor, thereby blocking wave propagation. Shielding effectiveness is enhanced by higher conductivity (for example, a copper shield is more effective than a steel or aluminum shield), and rendered less effective by any holes or slots in the surface of the shield. At higher frequencies a shield is less effective as conductivity is decreased due to the skin effect, and as the wavelength of the energy to be blocked approaches the dimension of any holes or slots in the shield. Although shielding with enclosures of this type can be quite effective, the resulting package is bulky and relatively expensive. Shielding may be further improved with the use of individually shielded conductors, i.e., coaxial cables. The principal difficulty with this approach is that it is costly to implement for every signal in a backplane. Backplanes often include 30-50 conductors and typically 10-16 connectors to the backplane, known as "drops." Replacing each conductor of the backplane with a coaxial cable would be prohibitively expensive.
A widely used shielding compromise involves the use of "modular" shields, wherein the backplane and each connected circuit module have separate shields. Modular shields are cost-efficient, but with the sacrifice that non-ideal shielding is provided, i.e., not as good as with the use of a single, encompassing conductive enclosure.
There is a direct relationship between the spectral content of data-carrying signals in electronic equipment and the electromagnetic interference (EMI) performance of the equipment. At lower baud rates there is less energy at the higher frequency ranges of the spectrum. Specifically, radiated power from a data-carrying conductor is reduced if the baud rate of the signal is lowered. A significant disadvantage that necessarily follows from reducing the baud rate is that the number of signal-carrying conductors must be increased if the same volume of data is to be transmitted. Control of radiated emissions can also be achieved by the use of a balanced transmission system where two conductors are twisted together and wherein the current carried in one conductor is returned by the other. Balanced transmission systems employing twisted conductors achieve EMI reduction by the tight, symmetric twisting of the conductors, which causes cancellation in the radiated fields. The obvious disadvantage of this technique is the requirement for two conductors per signal.
There is a well understood relationship between electromagnetic emission levels and the physical size of a circuit, acting as an antenna, that is responsible for the emissions. Specifically, larger circuits produce greater electromagnetic emissions. For example, in computer design the use of high frequencies is often limited to a small spatial area, such as that of a microprocessor chip, thus limiting the radiated emission levels. This option is not normally available in backplane design, because the backplane has to accommodate multiple plug-in modules, usually in the form of circuit boards. Therefore, as the need for higher bus speeds increases, there is an accompanying need for an alternative technique for reducing electromagnetic emissions.
It has been recognized in other contexts that the spectral content of data signals can be spread over a wider bandwidth, but with lower energy in each spectral increment, if the signals are modulated or "scrambled" with pseudorandom code. A pseudorandom code is one that changes in an apparently random fashion but actually repeats its sequence of changes over a relatively long time cycle. A pseudorandom code can be regenerated for use in a descrambling operation that recreates the original data signal. Scrambling techniques of this general type are used, for example, in SONET (Synchronous Optical Network), and in various satellite communication systems. Advantages of these techniques include the ability to recover clock signals from the received scrambled signals, because the energy of the data signals is distributed more uniformly across the frequency spectrum, and the reduction of radiated energy at any particular narrow frequency band.
It will be appreciated from the foregoing that there is still a need for improvement in the field of backplane design for interconnection of electronic equipment, especially communication equipment. In particular, an alternative technique is needed to reduce electromagnetic emissions from electronic equipment incorporating backplanes. The present invention satisfies this need.